1.

TC, SP A parallel beam is one which does not diverge or converge - its width stays the same as the wave (which could be light or microwaves for example) travels out from its source. Parallel beams are produced using curved (parabolic) reflector dishes (e.g. in car headlamps, satellite transmitter di [..]

2.

TC, SP This applies to light rays passing through any system of mirrors or lenses. Basically, it means if a ray of light follows a path when going, say, from left to right, then another ray of light going from right to left will follow the same path in reverse.

3.

HP, EL, EM, SP An atom is the smallest possible particle of any chemical element. For example, an atom of carbon is the smallest possible unit of carbon. All of the atoms of an element are identical to each other, but different from the atoms of any other element. For example, all carbon atoms are [..]

4.

UE An electrical appliance which is double insulated does not have an earth wire fitted. The appliance is designed in such a way that the electrical parts can never come into contact with the outer casing of the device. Common double insulated appliances are hair dryers, radios and cassette players [..]

5.

This is a phrase often uttered by those people who finish reading the zenith entry below, having read the whole of this file. Top Z Zenith SP The zenith is the point in the sky directly above an observer's head. This term is often used in astronomy.

8.

9.

UE Commonly known as the fusebox, the consumer unit is the part of the meter board which separates the incoming electrical supply into the various lighting and ring main circuits. Each of these circuits is protected by either a fuse or miniature circuit breaker (MCB)